This invention relates to a glass-ceramic substrate for a magnetic information storage medium used in an information storage device and, more particularly, to a glass-ceramic substrate for a magnetic information storage medium such as a magnetic disk substrate having a highly smooth substrate surface suitable for use in a near contact recording or a contact recording used mainly in the ramp loading system and also having a high Young""s modulus and low specific gravity capable of coping with a high speed rotation of the storage medium. The invention relates also to a method for manufacturing this glass-ceramic substrate for a magnetic information storage medium. This invention relates further to a magnetic information storage medium made by forming a film on such glass-ceramic substrate for a magnetic information storage medium. In this specification, the term xe2x80x9cmagnetic information storage mediumxe2x80x9d includes fixed type hard disks, removable type hard disks and card type hard disks used respectively such as so-called hard disks for personal computers and other magnetic information storage medium in the form of a disk which can be used for storage of data and can be also used in digital video cameras and digital cameras.
In contrast to conventional fixed type magnetic information storage devices, there have recently been proposed and put into practice magnetic information storage device of a removable type or a card type and developments of applications thereof for digital video cameras and digital cameras have been started. Such tendency relates to utilization of personal computers for multi-media purposes and prevailing of digital video cameras and digital cameras and, for handling data of a large size such as data of a moving image or voice, there is an increasing demand for a magnetic information storage device of a larger storage capacity. For coping with this demand, a magnetic information storage medium is required to increase its bit and track density and increase the surface recording density by reducing the size of the bit cell. On the other hand, as for a magnetic head, there is an increasing tendency toward adopting the near contact recording system and further the contact recording system according to which the magnetic head operates in closer proximity to the disk surface in accordance with the reduction of the bit cell size.
Aluminum alloy has been conventionally used as a material of a magnetic information storage medium substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness and smoothness as the above described magnetic information storage medium substrate of high recording density. Further, since the aluminum alloy is a soft material, its Young""s modulus and surface hardness are low with the result that a significant degree of vibration takes place during rotation of the disk drive at a high speed rotation which leads to deformation of the disk and hence it is difficult to cope with the requirement for making the disk thinner. Furthermore, the disk tends to be deformed due to contact with the magnetic head with resulting damage to the contents of the disk. Thus, the aluminum alloy cannot cope sufficiently with the requirement for recording with a high recording density.
As a material for overcoming the above problems of the aluminum alloy substrate, known in the art are chemically tempered glasses such as alumino-silicate glasses (SiO2xe2x80x94Al2O3xe2x80x94Na2O) disclosed in Japanese Patent Application Laid-open No. Hei 8-48537 and Japanese Patent Application Laid-open No. Hei 5-32431. This material, however, has the following disadvantages:
(1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner.
Further, the chemically tempered layer causes change with time after use for a long period of time with resulting deterioration in the magnetic property of the disk.
(2) Since the chemically tempered glass contains Na2O and K2O as its essential ingredients, the film forming property of the glass is adversely affected, and a barrier coating over the entire surface of the glass becomes necessary for preventing elution of Na2O and K2O ingredients and this prevents stable production of the product at a competitive cost.
(3) The chemical tempering is made for improving mechanical strength of the glass but this basically utilizes strengthening stress within the surface layer and the inside layer of the glass and hence its Young""s modulus is about 83 GPa or below which is about equivalent to ordinary amorphous glass. Therefore, use of the glass for a disk with a high speed rotation is limited and hence it is not sufficient for a magnetic information storage medium substrate.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some glass-ceramic substrates. For example, Japanese Patent Application Laid-open No. Hei 9-35234 and EP0781731A1 disclose glass-ceramic substrates for a magnetic disk made of a Li2Oxe2x80x94SiO2 system composition and has crystal phases of lithium disilicate and xcex2-spodumene, or crystal phases of lithium disilicate and xcex2-cristobalite. In these publications, however, the relation between Young""s modulus and specific gravity with respect to high speed rotation is not taken into consideration or suggested at all. The upper limit of Young""s modulus of these glass-ceramics is no more than 100 GPa.
For improving the Young""s modulus, Japanese Patent Application Laid-Open No. Hei 9-77531 discloses a glass-ceramic of a SiO2xe2x80x94Al2O3xe2x80x94MgOxe2x80x94ZnOxe2x80x94TiO2 system which contains a large amount of Spinel crystal as its predominant crystal phase and MgTi2O5 and several other crystals as its subordinate crystal phases and has Young""s modulus of 93.4 GPa to 160.11 GPa and a substrate for a high-rigidity magnetic disk made of this glass-ceramic. The glass-ceramic has, as its predominant crystal phase, Spinel crystal represented by (Mg/Zn) Al2O3 and/or (Mn/Zn)2 TiO4 containing a large amount of Al2O3 and has several other optional crystals as its subordinate crystal phaes. This glass-ceramic is significantly different from the glass-ceramic of the present invention which, as will be described later, contains a relatively small amount of Al2O3 and has a high Young""s modulus and a low specific gravity. Addition of such a large amount of Al2O3 is undesirable from the standpoint of production because it causes deterioration of melting property of the base glass and also deterioration of resistance to devitrification. Further, in this publication, the relation of Young""s modulus (GPa)/specific gravity and the value of specific gravity per se which are necessary for the high speed rotation are not taken into consideration or suggested at all. Particularly, the specific gravity in this publication is a high value of 2.87 or over. This publication therefore merely proposes a glass-ceramic substrate made of a rigid material. Moreover, the glass-ceamic of this system has the serious disadvantage that processability is poor and therefore is not suited for a large scale production since it is too hard and hence the improvement of this glass-ceramic as a high recording density magnetic information storage medium substrate is still not sufficient.
It is therefore an object of the invention to eliminate the above described disadvantages of the prior art materials and provide a glass-ceramic substrate for a magnetic information storage medium capable of coping with the tendency toward a high recording density recording, namely having excellent melting property, resistance to devitrification and processability, excellent smoothness of the surface of the substrate which is sufficient for contact recording of a high recording density and having a high value of Young""s modulus and a low value of specific gravity which are sufficient for a high speed rotation of the disk.
It is another object of the invention to provide a magnetic information storage medium made by forming a film of a magnetic medium on this glass-ceramic substrate.
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described objects of the invention have resulted in the finding, which has led to the present invention, that for coping with driving of the disk with a high speed rotation, a magnetic information storage medium substrate must have a high rigidity and a low specific gravity for preventing vibration of the disk caused by deflection and that an optimum ratio of Young""s modulus to specific gravity of the substrate is:
Young""s modulus (GPa)/specific gravity=37 to 63.
The inventors of the present invention have also found that a glass-ceramic substrate for a magnetic information storage medium can be produced which has a predominant crystal phase limited to one or more crystals selected from the group consisting of xcex2-quartz, xcex2-quartz solid solution, enstatite, enstatite solid solution, forsterite and forsterite solid solution, has grown crystal grains which are of a fine globular grain, has excellent melting property, resistance to devitrification and polishing processability, has superior flatness and smoothness in the surface after polishing, and has a high value of Young""s modulus and a low value of specific gravity capable of coping with driving of the disk with a high speed rotation.
For achieving the above described object of the invention, there is provided a high rigidity glass-ceramic substrate for a magnetic information storage medium having a ratio of Young""s modulus to specific gravity within a range from 37 to 63 and comprising Al2O3 within a range from 10% to less than 20%, and having a predominant crystal phase consisting of one or more crystals selected from the group consisting of xcex2-quartz, xcex2-quartz solid solution, enstatite, enstatite solid solution, forsterite and forsterite solid solution.
In one aspect of the invention, the high rigidity glass-ceramic substrate is substantially free of Na2O, K2O and PbO.
In another aspect of the invention, this substrate is provided by heat treating a base glass having a composition expressed on the basis of composition of oxides which consists in weight percent of:
In another aspect of the invention, a grain diameter of the respective predominant crystal phases is within a range from 0.05 xcexcm to 0.30 xcexcm.
In another aspect of the invention, a coefficient of thermal expansion within a range from xe2x88x9250xc2x0 C. to +70xc2x0 C. is within a range from 30xc3x9710xe2x88x927/xc2x0 C. to 50xc3x9710xe2x88x927/xc2x0 C.
In another aspect of the invention, the surface of the substrate after polishing has surface roughness (Ra) of 3 to 9 xc3x85 and maximum surface roughness (Rmax) of 100 xc3x85 or below.
In another aspect of the invention, the high rigidity glass-ceramic substrate is provided by heat treating the base glass at a nucleation temperature within a range from 650xc2x0 C. to 750xc2x0 C. and at a crystallization temperature within a range from 75xc2x0 C. to 105xc2x0 C.
In still another aspect of the invention, there is provided a magnetic information storage disk having a film of a magnetic medium formed on said high rigidity glass-ceramic substrate.
In this specification, the term xe2x80x9csolid solutionxe2x80x9d means each of the above described crystals a part of which is substituted by other ingredient or in which other ingredient has been mixed. The term xe2x80x9cxcex2-quartz solid solutionxe2x80x9d means xcex2-quartz crystal a part of which is substituted by an element other than Si or in which an element other than Si is mixed. xcex2-quartz solid solution includes xcex2-eucryptite (xcex2-Li2O. Al2O3. 2SiO2) in which Li and Al at the ratio of one to one partly substitute Si, xcex2-eucryptite solid solution which is xcex2-eucryptite crystal a part of which is substituted by other element or in which other element has been mixed, and a mixture thereof.
Reasons for limiting the physical properties, predominant crystal phases and crystal grain diameter, surface characteristics and composition of the glass-ceramics of the present invention will now be described. The composition of the glass-ceramics is expressed on the basis of composition of oxides as in their base glass.
Young""s modulus and specific gravity will first be described. As described previously, there has been an increasing tendency toward driving of a magnetic information storage medium with a high-speed rotation for improving the recording density and data transfer rate. It has been found by the inventors of the present invention that, for sufficiently conforming to such tendency, a material for a magnetic information storage medium must have high rigidity and low specific gravity in order to prevent vibration caused by deflection of the disk which occurs during high speed rotation of the disk, e.g., 10,000 rev/min. or over. A disk which has high rigidity but high specific gravity causes deflection during a high speed rotation with resulting vibration of the disk. It has been found that an apparently contradictory balance of physical properties of the material must be adopted, namely high rigidity and low specific gravity and this balance has been found to exist in the ratio of Young""s modulus (GPa) to specific gravity within a range from 37 to 63. A preferable range of Young""s modulus(GPa)/specific gravity is 40 to 63, a more preferable range thereof is 47 to 63 and the most preferable range thereof is 50 to 63. It has been also found that there is a preferable range of rigidity, That is, from the standpoint of preventing vibration, the material should preferably have rigidity of at least 120 GPa even if it has low specific gravity but, having regard to processability and increase of specific gravity, the upper limit of rigidity should preferably be 150 GPa. The same applies to specific gravity. That is, from the standpoint of preventing vibration, the material should preferably have specific gravity of 3.50 or less even if it has high rigidity, because otherwise vibration tends to occur during a high speed rotation due to its weight, but the material should preferably have specific gravity of 2.3 or over because otherwise it is difficult to obtain a substrate having desired rigidity. A more preferable range of specific gravity from these standpoints is 2.5 to 3.3.
If a material for a substrate contains Na2O or K2O, Na ion or K ion of these ingredient diffuses into a magnetic film during the film forming process when the temperature of the substrate rises to a high temperature (this phenomenon is particularly remarkable in a barium ferrite perpendicular magnetic film) and thereby causes abnormal growth of grains of the magnetic film and deterioration of orientation of the magnetic film. It is therefore important that the material of the substrate is substantially free of such ingredients. The material of the substrate should also be substantially free of PbO which is not desirable because of the environmental problems.
Description of. predominant crystal phases of the substrate of the invention will now be described.
A predominant phase of the substrate consists of one or more of the group consisting of xcex2-quartz, xcex2-quartz solid solution, enstatite, enstatite solid solution, forsterite and forsterite solid solution. This is because these crystal phases are advantageous in that they have excellent processability, contribute to increase in rigidity and to decrease in specific gravity and can have a very fine grain diameter of grown crystal. Growth and content ratio of xcex2-quartz, enstatite and forsterite are determined by the content ratio of MgO and SiO2. Growth and content ratio of these three crystal phases and solid solution of these three crystal phases are determined by the content ratio of MgO, SiO2 and other ingredients.
Description will now be made about crystal grain diameter of grown crystal and surface roughness. As was previously described, for coping with the near contact recording system and the contact recording system for improving the recording density, the smoothness of the surface of the magnetic information storage medium must be improved over the conventional magnetic medium. If one attempts to carry out high density inputting and outputting of information on the magnetic medium having the conventional level of smoothness, the distance between the magnetic head and the surface of the magnetic medium is so large that inputting and outputting of magnetic signals cannot be achieved. If one attempts to reduce this distance, the magnetic head will collide with projections of the magnetic medium resulting in damage of the magnetic head or magnetic medium. For coping with the near contact recording system and the contact recording system, it has been found that the substrate should have smoothness expressed in terms of surface roughness (Ra) of 3 to 9 xc3x85 and maximum surface roughness (Rmax) of 100 xc3x85 or below. Preferably, the surface roughness (Ra) should be 3 to 7 xc3x85 and the maximum surface roughness (Rmax) should be 95 xc3x85 or below. More preferably, the surface roughness (Ra) should be 3 to 6 xc3x85 and the maximum surface roughness (Rmax) should be 90 xc3x85 or below.
For obtaining a glass-ceramic substrate having a high rigidity and high flatness (3-9 xc3x85 in the data area) as in the present invention, the crystal grain diamter and crystal grain shape are important factors. It has been found that a substrate having a grain diameter which is larger or smaller than the range of diameter defined in the claims cannot achieve the desired strength and surface roughness.
In increasing the bit number and track density and reducing the size of the bit cell, difference in coefficient of thermal expansion between the magnetic film and the substrate significantly affects achievements of these objects. For this reason, it has been found that the coefficient of thermal expansion within a temperature range from xe2x88x9250xc2x0 C. to +70xc2x0 C. should preferably be within a range from 30xc3x9710xe2x88x927/xc2x0 C. to 50xc3x9710xe2x88x927/xc2x0 C.
In the glass-ceramic substrate according to the invention, the above described content ranges of the respective ingredients have been selected for the reasons stated below.
The SiO2 ingredient is a very important ingredient which, by heating a base glass, forms xcex2-quartz, xcex2-quartz solid solution, enstatite, enstatite solid solution, forsterite and forsterite solid solution crystals as predominant crystal phases. If the amount of this ingredient is below 40%, the crystal phases grown in the glass-ceramic are instable and their texture tends to become too rough whereas if the amount of this ingredient exceeds 60%, difficulty arises in melting and forming the base glass. For the growth of the crystal phases, conditions of heat treatment are also important factors. A preferable range of this ingredient which enables a broadened heat treatment conditions is 48.5-58.5%.
The MgO ingredient is a very important ingredient which, by heating a base glass, forms xcex2-quartz, xcex2-quartz solid solution, enstatite, enstatite solid solution, forsterite and foresterite solid solution as predominant crystal phases. If the amount of this ingredient is below 10%, the grown crystals of the obtained glass-ceramic are instable and their texture is too rough and melting property deteriorates. If the amount of this ingredient exceeds 20%, melting property and resistance to devitrification are reduced. For the same reason as in the SiO2 ingredient, a preferable range of this ingredient is 12-18%.
The Al2O3 ingredient is a very important ingredient which, by heating a base glass, forms xcex2-quartz solid solution as a predominant crystal phase. If the amount of this ingredient is below 10%, the grown crystals of the obtained glass-ceramic are instable and their texture is too rough whereas if the amount of this ingredient is 20% or over, melting property and resistance to devitrification are deteriorated. For the same reason as stated above, a preferable range of this ingredient is 12-18%.
The P2O5 ingredient functions as a nucleating agent for the glass and is also effective for improving melting property of the base glass and resistance to devitrification during molding. If the amount of this ingredient is below 0.5%, these effects cannot be obtained whereas if the amount of this ingredient exceeds 2.5%, resistance to devitrification is reduced. A preferable range of this ingredient is 1-2%.
The B2O3 ingredient is effective for controlling viscosity during melting and forming of the base glass. If the amount of this ingredient is below 1%, this effect cannot be obtained whereas if the amount of this ingredient exceeds 4%, melting property of the base glass deteriorates and the grown crystals of the glass-ceramic are instable and the texture becomes too rough. A preferable range of this ingredient is 1-3%.
The Li2O ingredient is a very important ingredient which, by heating a base glass, forms xcex2-quartz solid solution as a predominant crystal phase and improves melting property of the base glass. If the amount of this ingredient is below 0.5%, these effects cannot be obtained whereas if the amount of this ingredient exceeds 4%, the grown crystals of the glass-ceramic are instable and their texture become too rough. A preferable range of this ingredient is 1-3%.
The CaO ingredient improves melting property of the glass and prevents grown crystals from becoming too rough. If the amount of this ingredient is below 0.5%, these effects cannot be obtained whereas if the amount of this ingredient exceeds 4%, a desired crystal cannot be obtained and the grown crystals of the glass-ceramic become too rough and moreover chemical durability deteriorates. A preferable range of this ingredient is 1-3%.
The ZrO2 and TiO2 ingredients are very important ingredients whch function as a nucleating agent for the glass and also are effective for making the grown crystal grains finer and improving mechanical strength and chemical durability of the material. If the amount of the ZrO2 ingredient is below 0.5% and the amount of the TiO2 ingredient is 2.5%, these effects cannot be obtained. If the amount of the ZrO2 ingredient exceeds 5% and the amount of the TiO2 ingredient exceeds 8%, difficulty arises in melting the base glass and impurities such as ZrSiO4 are produced which are left unmolten and, moreover, resistance to devitrification is reduced. As a result, abnormal growth of crystal grains is brought about in the crystallizing process. A preferable range of the ZrO2 ingredient is 1-4% and a preferable range of the TiO2 is 3-7.5%. The total amount of the ZrO2 and TiO2 ingredients should preferably not exceed 9%. A more preferable range of the total amount of the ZrO2 and TiO2 ingredients is 3-8%.
The Sb2O3 and/or As2O3 ingredients may be added as a refining agent in melting of the glass. It will suffice if one or both of these ingredients are added up to the total amount of 2%.
The SnO2, MoO3, CeO and Fe2O3 ingredients may be added up to the total amount of 5% as a coloring agent or for improving sensitivity of detecting defects on the surface of the substrate by coloring the substrate and also improving laser absorption characteristics for an LD excited laser. As to MoO3, it will suffice if this ingredient up to 3% is added. The SnO2 and MoO3 ingredients are important in that they have light transmissivity in the glass state before the heat treatment and have the coloring property after the crystallization processing.
For manufacturing the glass-ceramic substrate for a magnetic information storage medium according to the invention, the base glass having the above described composition is melted, is subjected to heat forming and/or cold forming, is heat treated for producing a crystal nucleus under a temperature within a range from 650xc2x0 C. to 750xc2x0 C. for about one to twelve hours, and further is heat treated for crystallization under a temperature within a range from 750xc2x0 C. to 105xc2x0 C. for about one to twelve hours.